Fracture toughness of hot-pressed Lu2Si2O7–Si3N4 and Lu4Si2O7N2–Si3N4 ceramics and correlation to microstructure and grain-boundary phases

Lu2Si2O7–Si3N4 and Lu4Si2O7N2–Si3N4 composition ceramics were hot-pressed sintered. Microstructure was characterized by using scanning electron microscope and quantitative image analysis. The Lu2Si2O7–Si3N4 composition showed a fine-grained microstructure with average grain diameter size of 0.27 μm, whereas the grains of Lu4Si2O7N2–Si3N4 composition were coarser with an average grain diameter of 0.48 μm. On the other hand, fracture toughness of the two compositions was evaluated by means of Vickers indentation crack size measurements. Fracture toughness of Lu4Si2O7N2–Si3N4 composition was greater than that of Lu2Si2O7–Si3N4 composition. The greater fracture toughness for Lu4Si2O7N2–Si3N4 composition was predominantly caused by increase in grain size and volume fraction of needle-like Si3N4 grains compared with Lu2Si2O7–Si3N4 composition.